1. Field of the Invention
The present invention relates to a spread illuminating apparatus, and in particular a spread illuminating apparatus having a structure in which a light guide plate is housed in a housing frame.
2. Description of the Related Art
Currently, a sidelight-type spread illuminating apparatus is widely used as a backlight for liquid crystal display devices. This type of spread illuminating apparatus includes a light guide plate having a principal surface that is approximately the same size as a screen and a light source arranged on a side end surface of the light guide plate. By emitting light which has entered from the side end surface of the light guide plate from one of the principal surfaces, the screen can be uniformly illuminated. Such a spread illuminating apparatus often has a structure in which the light guide plate (and other constituent elements such as the light source and the like) is housed in a housing frame (refer to, for example, JP 2005-302485 A).
In many cases, such a spread illuminating apparatus is structured as follows: protrusions are provided on side surfaces of the light guide plate, and recesses are provided on side walls of the housing frame into which the protrusions are disposed, and when housing the light guide plate in the housing frame and disposing the protrusions of the light guide plate into the corresponding recesses, the light guide plate is positioned relative to the housing frame. Conventionally, since protrusions function as positioning and are provided on the side surfaces of the light guide plate, the bottom surfaces of the protrusions are usually made approximately flush with the bottom surface of the light guide plate.
FIG. 5 is a breakdown perspective view illustrating the spread illuminating apparatus disclosed in JP 2005-302485 exemplifying the structure of a conventional spread illuminating apparatus.
The spread illuminating apparatus M shown in FIG. 5 is used as a backlight of a liquid crystal panel N, and includes a light guide plate 101, point light sources 102, a reflecting sheet 103, a diffusing sheet 104, prism sheets 105 and 106, and a frame-shaped housing frame 107 in which these members are housed. The light guide plate 101 has an approximately rectangular shape, and protrusions 111a to 111c and 112a to 112c are provided on side surfaces 111 and 112 which are orthogonal to a side surface 108 at which the point light sources 102 are arranged.
The protrusions 111a to 111c and 112a to 112c are for positioning the light guide plate 101 relative to the housing frame 107. Recesses 120a to 120c and 121a to 121c, in which the respective protrusions 111a to 111c and 112a to 112c are arranged, are formed on side walls 122 and 123 of the housing frame 107.
In the spread illuminating apparatus M, the thickness of the protrusions 111a to 111c and 112a to 112c is formed to be thinner than that of the light guide plate 101. The protrusions 111a, 111c, 112a, and 112c are formed relatively at an emitting surface 109 side in a thickness direction of the light guide plate 101, and the protrusions 111b and 112b are formed relatively at a bottom surface 110 side of the light guide plate 101. In this structure, the protrusions 111b and 112b are exemplified that a bottom surface 113 thereof is made approximately flush with the bottom surface 110 of the light guide plate 101.
In accordance with the above-described structure of the protrusions 111a to 111c and 112a to 112c, the protrusions 111a, 111c, 112a, and 112c formed relatively at the emitting surface 109 side correspond to the recesses 120a, 120c, 121a, and 121c. In this case, there are recesses opened at a top surface 122a and 123a side (the emitting surface 109 side of the light guide plate 101). On the other hand, the protrusions 111b and 112b formed relatively at the bottom surface 110 side correspond to the recesses 120b and 121b. In this case, there are recesses opened at a bottom surface 122b and 123b side (the bottom surface 110 side of the light guide plate 101).
In recent, however, since the surface area of the light guide plate for the spread illuminating apparatus is demanded to increase, and to the contrary the thickness thereof is demanded to decrease, the following problems become prominent, which are also applicable to the spread illuminating apparatus M illustrated in FIG. 5. Here, in a finished product to which a liquid crystal panel is fixed, if the finished product is subjected to periodic temperature changes, cracks or fissures may occur at the root of the protrusions whose bottom surfaces are formed to be approximately flush with the bottom surface of the light guide plate (protrusions 111b and 112b in the example shown in FIG. 5) among the protrusions provided on the side surfaces of the light guide plate.
Cracks or fissures in the protrusions are not only undesirable in the structural stability of the spread illuminating apparatus, but may lead to uniformity losses in brightness of light emitted from the light guide plate thus decreasing visibility. This happens, particularly if cracks from the root of the protrusions entering toward the inside of the light guide plate extend toward the effective emitting area of the light guide plate.
Thus, based on considerable research, the present inventors identified why the problems have been caused, and thus successfully reached the present invention. Details will be discussed with reference to FIG. 6.
FIG. 6 is a partial cross-section showing the protrusion 111b portion of the spread illuminating apparatus M in an assembled state at a cross-section orthogonal to the extension direction of the side surface 111 that passes through the protrusion 111b. As shown in FIG. 6, in the spread illuminating apparatus M, the reflecting sheet 103 arranged on the bottom surface side 110 of the light guide plate 101 is normally retained on the housing frame 107 by adhering a peripheral edge thereof to the bottom surface 122b of the side wall 122 of the housing frame 107 via an adhering means such as a double-sided tape 125.
FIG. 6 shows a step into which the reflecting sheet 103 is introduced, the reflecting sheet 103 being provided between an outer edge part 122c and an inner edge part 122d of the bottom surface 122b of the side wall 122 (this step is omitted in FIG. 5). In FIG. 6, the bottom surface 113 of the protrusion 111b in the light guide plate 101 has a constant step to the bottom surface 110 of the light guide plate 101. However, this is for emphasizing the negligible small step based on the thickness of the double-sided tape 125. Thus, in actual conditions, the bottom surface 113 of the protrusion 111b is approximately flush with the bottom surface 110 of the light guide plate 101.
Normally, the double-sided tape 125 is continuously adhered without keeping off from the recesses 120a to 120c provided on the bottom surface 122b (the inner edge 122d in the case of the illustrated example) of the side wall 122. Therefore, a portion of the recess 120b which is formed to be opened at the bottom surface 122b side is covered by the double-sided tape 125, and at the same time, the double-sided tape 125 is also adhered to the bottom surface 113 of the protrusion 111b disposed in the recess 120b. 
The protrusion 111b is thus restrained to the reflecting sheet 103 and the housing frame 107 via the double-sided tape 125. It was found that if the spread illuminating apparatus M is subjected to periodic temperature changes, the protrusion 111b is repeatedly subjected to stress caused by a difference in the thermal expansion coefficient of the constituent members of the spread illuminating apparatus M, and this can easily lead to cracks or fissures at the root of the protrusion 111b at which the stress is concentrated. (In fact, if the portion of the double-sided tape 125 that covers the recess 120b is intentionally removed, cracks or fissures did not occur at the root of the protrusion 111b.)